(a) Field of the Invention
The present invention relates to a method for arranging minute metallic electrodes to form an array of bump electrodes for use in BGA (ball grid array), CSP (chip size package), and flip chip bonding techniques.
(b) Description of the Related Art
It is essential to arrange a higher number of external pins at a smaller pitch in view of the demand for a higher mounting density, lower device area and increasing capacity of the integrated circuits. Bump electrodes are generally used for the external pins using BGA, CSP and flip chip bonding techniques. Thus, in the method for forming an array of bump electrodes by arranging minute metallic balls or solder balls, it is important to arrange a large number of minute metallic balls efficiently at a small pitch. Some proposals have been made using arrangement tools or templates for arranging the metallic balls.
Referring to FIG. 1A showing a first conventional technique, proposed in JP-A-8(1996)-330716, an arrangement tool or template 19 is set up at a speed position on a mounting board 18 having terminals 24 thereon. The setup is obtained by aligning four guide pins 22 of the mounting board 18 with respective positioning holes 23 of the template 19. The template 19 has through-holes 20 for receiving solder balls (metallic balls) 21 at the positions corresponding to the terminals 24 on the mounting board 18. After the metallic balls are scattered on the template 19, some of the solder balls are received in the through-holes 20. A squeegee is used to scan the template 19 for receiving the remaining solder balls 21 in the through-holes 20 and for removing excess solder balls 21, thereby filling the through-holes 20 with the solder balls 21, as shown in FIG. 1B. The solder balls 21 mounted on the terminals 24 of mounting board 18 in the through-holes 20 are heated in a reflow furnace, and melted to form an array of solder bumps on the mounting board 18.
In the technique as mentioned above, it is possible to accurately position the template 19 and accordingly to accurately position the solder balls 21 onto the terminals 24. However, electrostatic charge is often generated on the solder balls 21 by the friction between the solder balls 21 or between the solder balls 21 and the template 19, which results in cohesion of the solder balls 21. It involves therefore a problem in that two or more of the solder balls 21 enter a single hole 20 of the template 19 to reduce the yield of the process. The cohesion of solder balls 21 may also be generated due to moisture in the atmosphere.
Referring to FIG. 2 showing a second conventional technique, descried in JP-A-6(1994)-310515, solder balls (metallic balls) 28 are supplied onto a feeder plate 27 in which a plurality of holes each for receiving a solder ball 28 are arranged. Each of the solder balls 28 is attracted to the bottom of the hole with vacuum formed by a vacuum pump 40. Excess solder balls 28 are ejected through an outlet 30 by raising a side cover 29 and supplying vibration from a vibrator 31. The feeder 27 is then transferred to the right, wherein ion air 48 is provided from an ion generator 47 onto the surface of the solder balls 28 and the feeder plate 27, which are often electrified due to friction by the vibration, to electrically neutralize the surface of the solder balls 28 and the feeder plate 27.
Compressed inert gas 46 is then supplied from a nozzle 45 to eject the neutralized and excess solder balls 28 through the outlet 30. The use of inert gas 46 prevents occurrence of adhesion, cohesion, bridging, oxidation and electrostatic charge in the solder balls 29. After the ejection, the feeder plate 27 is then subjected to inspection using a TV camera 44 to assure whether all the holes are filled with the solder balls 28. If there is any defect wherein a hole is not supplied with a solder ball or a plurality of solder balls are attached to a single hole, the feeder plate and the solder balls are not supplied to the next step.
In the second conventional technique, there is a problem in that the neutralization is not uniform among the solder balls 28 depending on the distance from the ion source 47 and the solder balls 28, which may cause electrostatic charge remaining on some of the solder balls 28. In addition, in the case of minute solder balls 28 having a diameter as low as 100 .mu.m or less, a solder ball 28 normally received in a hole may be removed by the ion air or the compressed inert gas. The minute solder balls 28 may cohere also due to moisture.
Referring to FIG. 3 showing a third conventional technique, described in JP-A-4(1992)-75357, a template 56 has a pluarality of holes 57 arranged thereon which are communicated with a vacuum chanter 60 including therein a vacuum pump and an exhaust tube 59. Minute metallic balls 54 supplied onto a table 55 are attracted toward the template 56 while irradiated with ion beams supplied from an ion gun or ion blower 61. The minute metallic balls 54 each attracted to the array of the holes are transcribed to a semiconductor chip or a mounting board not shown. The third conventional technique has also problems similar those in the second conventional technique.
As described above, conventional techniques generally involved similar problems during arranging minute metallic balls.